Near Field Communication (i.e., NFC) technology has recently become more popular for use in cellular phones in the background of the rapid growth of the Radio Frequency Identification (RFID) market. This technology opens up many new possibilities for cellular phones, for example, enabling the cellular phones to have the function of electronic keys, an ID card and an electronic wallet, and also enabling the exchange of phone numbers with other people to be done in a quick manner via wireless channels.
NFC is based on a 13.56 MHz RFID system which uses a magnetic field as carrier waves. However, the designed communication range may not be attained when a loop antenna is close to a metal case, shielded case, ground surface of a circuit board, or sheet surfaces such as a battery casing. This attenuation of carrier waves occurs because eddy current induced on the metal surface creates a magnetic field in the reverse direction to the carrier wave. Consequently, materials, such as Ni—Zn ferrites (with the formula: NiaZn(1-a)Fe2O4), with high permeability that can shield the carrier wave from the metal surface are desired.
In typical NFC applications, an electronic device collects the magnetic flux circulating around a loop reader antenna. The flux that makes it through the device's coils excites a voltage around the coil path. When the antenna is placed over a conductor, there will be a dramatic reduction in magnetic field amplitudes close-in to the surface. For a perfect conductor, the tangential component of the electrical field is zero at any point of the surface. As a result, the presence of metal is generally detrimental to RFID tag coupling because there will be no normal component of the magnetic field at the conductor surface contributing to the total flux through the coil. According to Faraday's law, there will be no voltage excitation around the coil. Only marginal thickness of the dielectric substrate of the antenna allows small magnetic flux through the tag.
The detrimental effect of a metal surface near the antenna can be mitigated by putting a flux field directional material (i.e., a magnetic isolator) between the metal surface and the tag. An ideal high permeability magnetic isolator will concentrate the field in its thickness without making any difference in the normal magnetic field at its surface. Ferrite or other magnetic ceramics are traditionally used for this purpose because of their very low bulk conductivity. They show very little eddy current loss, and therefore a high proportion of magnetic field remains normal through the antenna loop. However, their relatively low permeability requires higher thickness of the isolator layer for efficient isolation, which increases cost and may be problematic in microminiaturized devices.
Nanocrystalline soft magnetic materials may supersede powdered ferrite and amorphous materials for high-frequency applications in electronics. In the last two decades, a new class of bulk metallic glasses with promising soft magnetic properties prepared by different casting techniques has been intensively investigated. Among the several developed metallic glass systems, Fe-based alloys have attracted considerable attention due to their good soft magnetic properties with near-to-zero magnetostriction, high saturation magnetization, and high permeability.
Among different Fe-based alloys, amorphous FeCuNbSiB alloys (e.g., those marketed by VACUUMSCHMELZE GmbH & Co. KG, Hanau, Germany, under the VITROPERM trade designation) are designed to transform into nanocrystalline material when annealed above 550° C. The resultant material shows much higher permeability than the as-spun amorphous ribbon. Due to the inherently conductive nature of the metallic ribbon, eddy current losses from the isolator can be problematic. In one approach to reducing eddy current loss, the annealed nanocrystalline ribbon has been placed on a carrier film and cracked into small pieces.
Eur. Pat. Appl. Publ. 2 797 092 A1 (Lee et al.) describes a magnetic field shield sheet for a wireless charger, which fills a gap between fine pieces of an amorphous ribbon through a flake treatment process of the amorphous ribbon and then a compression laminating process with an adhesive, to thereby prevent water penetration, and which simultaneously surrounds all surfaces of the fine pieces with an adhesive (or a dielectric) to thus mutually isolate the fine pieces to thereby promote reduction of eddy currents and prevent shielding performance from falling, and a manufacturing method thereof.